1. Field of Invention
The present invention relates to electronic and inductive communication apparatus'. Specifically, the present invention relates to a transponder, more particularly, a passive transponder. The passive transponder may be inductively powered and may store information or perform electronic functions when it is so powered. The transponder of the present invention relates to a portable, integrated and relatively cheap apparatus advantageously adapted for interrogation and/or identification of an article with which the transponder is associated. The transponder of the present invention advantageously utilises a single coil transmission and reception system. Furthermore, the unique circuit arrangement(s) of the present invention provides a single component rectifying means. The present invention lends itself to integration in a single chip form. The means used to receive a power providing signal may also be used to transmit another signal, the reception and transmission occurring in a simultaneous manner.
2. Description of the Related Art
The Applicant is aware of U.S. Pat. No. 3,859,624, which discloses two types of transponder used within an identification tag system. One form of the transponder disclosed therein includes an inductive powering field receiving coil and a separate coplanar coded information field generator coil. The power receiving coil has associated therewith a rectifier, regulator and energy storage device. The separate information coil is used as a transmitter means to an interrogation station. Each coil operates independently of the other.
The identification tag system further includes an interrogation station comprising an inductive power field generator, for the first type of tag discussed above, and an information code receiver. The interrogation station disclosed utilises a unitary coil for both the power field generation and the coded information receiver. The interrogator means, however, operates in a sequential and cyclic fashion. A first mode comprises AC power (inductive) generation by the interrogator for a finite time. A second mode, during which no AC (inductive) power is radiated to the transponder, the interrogator operates as a receiver to receive a coded information signal for a finite time. These modes are performed continuously and sequentially. No disclosure exists of simultaneous power and data reception with data transmission.
There are a number of patents utilising the principle outlined most clearly by Vinding U.S. Pat. No. 3,299,424 but in face first detailed by Brard U.S. Pat. No. 1,744,036.
In U.S. Pat. No. 3,299,424, power is radiated from a transmitter (or interrogator) and received by a tuned circuit in the transponder. The power signal induces a current to flow in the transponder's tuned circuit. This current radiates a transmission signal from the transponder's tuned circuit which is detected at the interrogator. By varying any parameter of the tuned circuit (such as tuning or loss) the induced current's phase or magnitude can be caused to vary. Thus coded data modulating a parameter of the transponder's tuned circuit can be detected and decoded by suitable circuitry at the interrogator.
The crucial point is that the current induced in the transponder's tuned circuit, by the interrogator, generates the reply signal that carrier data back to the interrogator. The "carrier" frequency for this data signal is almost invariably the same as the interrogator's power signal. A number of disclosures rely upon the generation of sub-harmonic currents, by a suitable switch means, from the current induced in the transponder's tuned circuit, i.e. Harris U.S. Pat. No. 2,979,321 and Sellers U.S. Pat. No. 4,314,373 which cannot operate with fewer than two tuned circuits. Once again the actual current in the transponder's tuned circuit directly generates the transmission signal for carrying data.
The distinctive difference is that the carrier signal is not injected into the pickup coil. This limits the transmission signal to be the same frequency or a sub-harmonic of the interrogation signal and precludes the simultaneous reception and transmission of data by the transponder's tuned circuit.
Other disclosures which essentially operate in the same or a similar manner to that of Vinding U.S. Pat. No. 3,299,424 include: U.S. Pat. Nos. 4,075,632; 4,196,418; 4,333,073; 4,361,153; 4,546,241; 4,580,041; and 4,654,658.
U.S. Pat. No. 4,040,053 relates to a microwave system. Power is transmitted in high frequency pulses. The pulse frequency is used as the timebase reference for the transponders internal logic. Without the pulses the transponder's internal logic cannot be clocked. The present invention can either directly use the period of the power field as a clock reference or, in a preferred form, derive the clock from an internal oscillator. If the external interrogation signal is momentarily absent proper clock signals will still be generated by the oscillator. For U.S. Pat. No. 4,040,053 the reply transmission can only occur during the interrogator's power phase. When the pulse is absent no reply carrier is generated. This reply carrier frequency is fixed at a preset harmonic of the interrogation frequency and cannot be varied at will. Data cannot be transmitted to the transponder because the circuit uses the pulse signals to clock the internal logic. All data is preprogrammed onto the transponder.
It should be noted that in U.S. Pat. No. 4,040,053 data transmission by the transponder does and can only occur simultaneously with the power pulse from the interrogator. The interrogation signal is frequency doubled and reradiated back at the interrogator. Rather than the transmission reply signal being generated and injected into the antenna by the transponder electronics circuitry.
Three tuned circuits are required to receive power and generate the information transmission. There are power rectifiers and a "frequency translation" device which are not integratable.
In U.S. Pat. No. 4,730,188 the transponder described utilises FSK data transmission at integer sub-harmonics of the interrogation frequency. The circuits disclosed are not integrable and no provision for the reception of data is made.
The circuit disclosed uses a full wave bridge rectifier to convert the received AC voltage in the PIT coil to DC voltage for the transponder electronics. Full wave rectifiers are considered impossible to integrate using commercial NMOS, CMOS or, at the power levels required, bipolar processing lines. This contrasts with the rectifying structure(s) employed by the present invention which may be readily integrated, whether a diode, synchronous rectifier, or other rectifier means is employed.
Data is outputted as a binary FSK data stream both frequencies being a subharmonic of the power field. No provision is made to generate frequencies greater than the power field's frequency. This contrasts with the special phase coherent frequency multiplier which may be used in the present invention and which allows higher frequencies to be generated, transmitted and coherently detected. High frequencies couple more efficiently from the transponder back to the interrogator.
The adoption of FSK signalling results in a far wider spectral spread on the transmission data than direct modulation (BPSK or QPSK for example) used by the present invention. The two FSK carriers are envelope modulated by the data and data complement respectively. The data spectrum convolves with each FSK carrier. The total data stream bandwidth is double that of any single carrier system. The extra bandwidth required to receive the data signal degrades the systems noise and interference performance.
A data signal is transmitted from the PIT using two open collector output stages each with a series current limiting resistor in their respective collector circuits. These resistors are connected to opposite sides of the PIT. The transistors are driven with complimentary signals and are driven hard "on" forcing them to act as switches. The resistors serve to limit the current flow from the PIT. The dual complimentary drive stage is provided to force signal currents through the PIT coil. Proper operation of this output drive stage is dependent upon two factors, namely the bridge rectifier being connected across the PIT, and the carrier frequency being less than the powering frequency.
The bridge rectifier operates to ensure that the voltage between each side of the PIT and Vss is either zero or greater than zero. During each half cycle the bridge connects alternate sides of the coil to Vss and Vdd respectively. The voltage on the coil side connected to Vdd is a half sinewave. The currents drawn through 4R1 and 4R2 are likewise half sinewaves. The total current through the PIT consists of half sinewave pulses directed through whichever output resistor (4R1, 4R2) is active.
With the outputs driven by FSK the signal current through the PIT consists of a burst of half sine current pulses through 4R1 followed by a burst of half sine current pulses through 4R2. The period of these bursts being determined by the period of the keying frequency.
Clearly this system is incapable of accurately transmitting a frequency higher than the powering frequency without tolerating amplitude modulation of the data by the PIT voltage. Such heavy high frequency amplitude modulation makes detection and demodulation of the data a complex and difficult process. The present invention may inject any frequency(s) of current (both higher or lower than the power field's frequency) without any modulation, amplitude or otherwise, of the injected current due to the voltage induced in the antenna coil by the powering field. Any choice of transmission modulation (amplitude, FSK, phase etc.) can be used with complete disregard for the interrogation signal.
The external bridge rectifier and series connected resistors (4R1, 4R2) of U.S. Pat. No. 4,730,188 are crucial to the operation of that transponders communication channel. Neither of these components have been disclosed in an integrated form nor in a readily integratable form. The rectifier and current source advantageously sued in the present invention lend themselves to integration within a fully customised integrated circuit.
With particular reference to line 23 in column 3 the transponder outputs are transistor switches that "sink" current through their respective series collector resistors. The output stage is not a current source, a current source being an element which constrains the current independently of the voltage across it. The output current is grossly affected by the PIT voltage, i.e. heavily amplitude modulated. The series collector resistors provide the only current restricting function. The circuit is incapable of envelope shaping the data transmission bandwidth to bandlimit the transmission signal.
In the present invention, by the careful selection of the output transistor current source, rectifier and antenna configuration, far fewer components are required for power reception and data transmission, i.e. only 1 transistor and 1 rectifier versus 2 transistors, 2 resistors and 4 rectifiers of the prior art.
The maximum voltage at any part of the PIT coil is (Vdd-Vss) i.e. peak pickup voltage. Whereas the voltage point in the present invention has a voltage of twice the peak voltage. This is ideally suited as a programming voltage for electrically erasable memory (EEPROM) which requires a high programming voltage. The circuit of the present invention automatically supplies this high programming voltage if it is required.
U.S. Pat. No. 4,724,427 describes a transponder that utilises a single antenna coil to receive power and transmit data.
The central feature of this disclosure is the use of a specially designed diode bridge (4 diodes minimum--5 diodes are actually used) to simultaneously perform the tasks of: (1) Rectification, and (2) Modulation. The bridge acts to rectify the power received by the transponder's antenna. Additionally, it can be caused to "mix" an encoded data signal with the power frequency. This "mixing" action results in the generation of new frequencies at the sum and difference between the power frequency (fc) and the encoded data frequency (fd), where fd=fc-n, i.e., fc-fd and fc+fd.
Unlike the present invention those embodiments of U.S. Pat. No. 4,724,427 using a single antenna do not have power reception and information transmission occurring simultaneously and independently. Data carrying sum and difference frequencies are only generated by the action of both the power frequency and the encoded data frequency in the diode bridge. Data cannot be transmitted unless the power signal is present. The frequencies available for data transmission are constrained to be near the carrier frequency i.e., fc.(1+1/n), fc(1-1/n).
Modulation of the power signal to programme data/commands into the transponder cannot occur simultaneously with the transmission of data by the transponder. The modulation sidebands in the power signal mix with the transponder's data sidebands in the diode bridge mutilating the transponder's data, and possible the power modulation. Additionally, the data clock is directly derived from the power frequency. Frequency or phase modulation of the carrier will similarly modulate the data clock further corrupting the sum and difference sidebands.
A principle distinction between U.S. Pat. No. 4,730,188, U.S. Pat. No. 4,724,427 and the present invention is that the prior art requires the carrier signal to be present for the generation of the data transmission. The present invention not only does not require the power signal to generate the information transmission signal, but can transmit the signal with and independently of the powering signal.
U.S. Pat. No. 4,724,427 presents a diagram in which the transponder antenna and circuitry are incorporated onto a single chip. While such a small transponder would find many uses it suffers from the deficiency of not being reprogrammable "on the fly" i.e. while being interrogated.
Such an ability of highly advantageous for such applications as smart cards where frequent infield reprogramming occurs as a matter of normal operation.